Could a Japan-like earthquake happen in the United States?

The short answer is yes, most likely along the Pacific Northwest coast.

And at the risk of being accused of scare-mongering, I will say it’s not unrealistic to contemplate such a possibility in our lifetimes.

A Japan-like event — a magnitude 9.0 earthquake offshore the United States that creates a large tsunami — would most likely occur in the Cascadia region off the coast from Northern California to Vancouver Island.

This is where the Juan de Fuca plate slips under the North American plate. In essence, the ocean floor is being pushed beneath the continent off the coasts of Oregon and Washington (illustrated below). This region is known as the Cascadia subduction zone.

Pacific Northwest Seismic Network

This seafloor material heats up as it is pushed deeper beneath the North American continent. It becomes more molten, and stressed and eventually generates earthquakes. The world’s largest earthquakes — magnitude 9.0 and above — form in these great subduction zones.

And it has happened before off the coast of the northwestern United States. In the year 1700 an earthquake with an estimated magnitude of 9.0 shook the Pacific Northwest, producing a tsunami in excess of 30 feet along the Washington coast by some estimates. It also produced a 10-foot tsunami in Japan.

Such events are also not particularly rare. Geological evidence indicates that great earthquakes may have occurred at least seven times in the last 3,500 years, suggesting a return period of 400 to 600 years for a megathrust.

Although building codes along the northwestern United States coast are quite good, we have very recently seen the effects of a large earthquake paired with a large tsunami striking a prosperous, developed country. Seattle and Vancouver, British Columbia could be devastated.

There are presently scientific efforts afoot to better understand the Cascadia subduction zone, and improve monitoring. But there’s nothing we can do to prevent the next megathrust from coming.

My thanks to John of John’s blog for his assistance in preparing this blog entry.